Several industrial applications involve the need to measure the level in a container or a tank, filled with substances such as liquid or bulk material. There are several techniques for determining the fill level which can be split in two main categories depending on whether the sensing element is in contact with the substance or not. Non-intrusive and non-contact techniques have several advantages, notably in terms of contamination and maintenance. Radar and ultrasonic technologies are generally involved in non-contact level measurement. However, radar technology has some limitations when used with low dielectric material and is sensitive to false echoes. Ultrasonic technology shows lower performance when pressure and temperature fluctuate.
Optical level measurement is another non-contact technique. This technique uses a light beam which is directed to the top surface of the liquid or material and reflected back to an optical sensor. For liquid level measurement, a floatable reflector is generally used to improve the performance. Some optical methods use the speed of light to determine the distance between the sensor and the surface. One of such methods is based on the time of flight principle using short pulses of light generated by a laser.
Optical systems based on the time of flight principle are well suited for fill-level sensing applications where the range of the detection is relatively short (typically <100 m) and the refresh rate is relatively low. However resolution and accuracy should be relatively high. In order to provide such accuracy, such optical level systems usually use costly lasers with very short pulses (few ns or less).